Support insulator

ABSTRACT

The electrical insulator has an insulator body  3  which is fitted between an electrical conductor  1  and a grounded holder  2 . The surface of the insulator is at least partially formed by a protective body  6 . The material of the protective body has a low dielectric constant in comparison to that of the material of the insulator body  3 . The protective body  6  prevents an electrically conductive particle  12  from coming to rest directly on the surface of the insulator body  3 , in particular in the region of the triple points T, or causing a considerable increase in the field due to immediate proximity to the insulator body  3.    
     The breakdown voltage of a gas-insulated system which contains such insulators provided with a protective body is increased. Gas-insulated systems can thus be made more compactly and more cheaply, and their life can be extended.

This application is a National Stage filing under 35 U.S.C. §371 ofInternational Application No. PCT/CH00/00210 having an InternationalFiling Date of Apr. 11, 2000, the entire contents of which areincorporated herein in its entirety.

TECHNICAL FIELD

The invention is based on an electrical insulator for supporting anelectrode, which carries high voltage, on a holder as claimed in theprecharacterizing clause of patent claim 1.

Such an insulator is used, for example, in medium-voltage andhigh-voltage technology for supporting a live electrical conductor withrespect to grounded parts of a system.

PRIOR ART

An electrical insulator of the type mentioned initially is described,for example in DE 40 07 337 A1. In this insulator, surfaces of aninsulated body are coated with an insulating glazing compound with ahigh dielectric constant for field control.

The surface of the insulating body is a dielectric weak point of theinsulator. Electrically conductive particles, such a metal swarf, whichenters the encapsulation during installation, or particles detached fromthe conductor, which can be formed by thermal cycling and the movementresulting from it, have the tendency to adhere to the surface of theinsulator body, owing to the high dielectric constant of said insulatorbody. Such particles lead to a considerably reduced breakdown voltage,since the electric field around the particles is increased by theproximity of the insulator body, resulting in a field peak. A criticalzone with regard to flashovers is formed in the region of this fieldpeak.

An electrode for controlling electric fields in a gas-insulated area isdescribed in WO 98/22958. This electrode has an electrically conductivesurface and is at least partially coated with a material which has a lowdielectric constant. The material contains, for example, a porous foamwhich, thanks to the enclosed gas, has approximately the same dielectricconstant as the surrounding gas, but with a dielectric strength which isgreater than that of that gas.

A coating of an insulating supporting element for electrodes, which aredirectly seated or arranged at a certain distance, for high-voltageswitchgear assemblies is known from DE 3140652. This coating is composedof a material having a dielectric constant which is less than that ofthe material of the supporting element.

BRIEF DESCRIPTION OF THE INVENTION

The invention, as it is specified in patent claim 1, is based on theobject of providing an electrical insulator of the type mentionedinitially which, despite having a compact construction, is distinguishedby high operational reliability when the dielectric load is high.

The surface of the insulator with an insulator body is at leastpartially formed from a protective body. This has a structure whose meandielectric constant is less than that of the material of the insulatorbody. This makes it possible to avoid an excessive field peak in theregion of a conductive particle located on the surface of the insulator.In order to achieve a field peak which is as low as possible, theprotective body should have a thickness which corresponds to the maximumlength of the particles to be considered.

In a first embodiment, the protective body contains a foam. In contrastto the coatings for insulator bodies which are known from the prior art,a gaseous medium surrounding the insulator penetrates into the pores ofthe foam, thus resulting in the foam having a mean dielectric constantwhich virtually corresponds to that of the gaseous medium. Syntacticfoams are particularly advantageous, since the size of the pores can becontrolled well, and this is of major importance for the dielectricstrength of the material.

In a further embodiment, the protective body is in the form of ahoneycomb, or is formed from a large number of thin walls arrangedparallel to the lines of force. The thin walls and the small contactsurface areas associated with them advantageously result in smallcritical zones.

If the insulator is attached to a grounded holder and/or supports anelectrical conductor, then it is particularly advantageous to use aflexible or elastic foam. This is guided along the conductor and/or thegrounded holder, and thus prevents the ingress of particles into any gapwhich may be present in the region of the triple point between theinsulator body and the holder.

A gas-insulated system having such insulators can be produced morecompactly and more cheaply and can be designed for higher voltageswhile, at the same time, having a longer life expectancy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following textwith reference to drawings, in which:

FIGS. 1 to 3 show a view of an axial section through the upper part ofan arrangement with three embodiments of the insulator according to theinvention, FIG. 4 shows a view of a part of the arrangement, shownenlarged, from FIG. 3, FIG. 5 shows a view of an axial section through atest apparatus with an insulator according to the prior art, and FIG. 6shows a view of an axial section through a test apparatus with aninsulator according to the invention.

APPROACH TO EMBODIMENTS OF THE INVENTION

Identical reference symbols denote parts having the same effects in allthe figures.

FIG. 1 shows an arrangement having an electrical conductor 1,encapsulation 2 which surrounds the conductor, is grounded, is tubularand pressure-resistant, and is formed from an electrically conductivematerial, and an insulator body 3, which is in the form of a disk and iscomposed of epoxy materials or other solids with a relatively highdielectric constant (greater than 4.0). The insulator body 3 rests onthe encapsulation 2. There is a cavity 7 between the conductor 1 and theencapsulation 2, and this is filled with a gaseous medium, for exampleSF₆, at atmospheric or increased pressure.

A protective body 6, which at least partially forms the surface of theinsulator, is fitted on the insulator body 3. This protective body 6contains a structure having a mean dielectric constant which is lessthan that of the material of the insulating body 3, preferably in thevicinity of unity. This protective body may, for example, contain a foamwith open pores which, thanks to gas being exchanged with the gaseousmedium located in the cavity 7, has approximately the same dielectricstrength as the medium itself. A foam with closed pores must have anadequate dielectric strength. The foam may also be syntactic, that is tosay it may be composed of small hollow spheres which are either sinteredto one another or are arranged in a solid matrix. The size of the poresin the syntactic foam may be varied. Small pores allow a high dielectricstrength to be achieved. Furthermore, the material may also containaerogels, in particular composed of silicone, which are distinguished byvery low dielectric constants and, thanks to the porous structure, bygood dielectric strength.

The object of the protective body 6 is to prevent electricallyconductive particles 12 from adhering directly on the insulator body 3,in particular in the region of the triple points T in the region of thejunctions between the insulator on the conductor 1 and the encapsulation1. The area of the insulator body 3 with the relatively high dielectricconstant would lead to an increase in the electric fields in theimmediate vicinity of the particles 12. Thanks to the protective body 6with the low dielectric constant, the field peaks in the vicinity of theparticles 12 are lower.

The protective body 6 has a thickness of at least 1 mm, whichcorresponds to the typical maximum length of the particles to beconsidered.

The protective body 6 may be bonded on the insulator body. This ispreferable to manufacturing a two-component insulator body with acomparable dielectric strength. The protective body may be bonded on atpoints, which is less complex than area bonding.

Furthermore, a thinner cavity may be provided between the insulator body3 and the protective body 6, provided it is certain that there are noelectrically conductive particles in the cavity.

In one preferred embodiment of the insulator according to the invention,as shown in FIG. 2, the protective body 6 covers any gaps 8 which may bepresent between the insulator body 3 and the encapsulation 2, in theregion of the triple points T. The protective body 6 advantageouslycontains a flexible foam, for this purpose. The protective body 6 is apart of the encapsulation 2, and is drawn along the conductor 1. Thismakes it possible to prevent electrically conductive particles 12 fromentering the gaps 8, where they would adversely affect the dielectriccharacteristics of the insulator in the region of the triple points T.

In a further embodiment of the insulator according to the invention andas shown in FIG. 3, a protective body 6 is arranged between theinsulator body 3 and the encapsulation 2, and/or between the insulatorbody 3 and the conductor 1. As illustrated in enlarged form in FIG. 4,this protective body may be in the form of a thin-walled, honeycomblayer 6′. Instead of honeycombs, vertical thin walls or supports mayalso be provided. The important factor is that the thickness of thesewalls is less than the typical length of the electrically conductiveparticles, which could lead to a discharge.

The higher breakdown voltage of an electrically conductive particle on aprotective body having a low dielectric constant has been verifiedexperimentally. FIG. 5 and FIG. 6 show a test apparatus with twoelectrodes 10 and 20, one of which is connected to ground, while thetest voltage (high voltage) U is applied to the other. A conventional,uncoated epoxy post 3 is fitted between the electrodes 10 and 20, and isintended to simulate an insulator body. For a first test (FIG. 5), a 4mm-long, electrically conductive particle 12 is bonded in the center ofthe post 3. For a second test (FIG. 6), a protective body 6 in the formof a 6 mm-thick layer, with a 4 mm-long particle 12 on it, is fixed inthe center of the post 3. The test apparatus is surrounded by insulatinggas (SF₆) at increased pressure (5 bar).

The second test, with the layer on the support, resulted in thebreakdown voltage being increased by around 75% in comparison to thefirst test without any coating. The protective body on the insulatorbody thus allows the hazard presented by electrically conductiveparticles to be reduced considerably.

In addition to being used in gas-insulated systems, such insulators canalso be used in other encapsulated systems, in particular those withnon-conductive encapsulation, or in non-encapsulated systems, inparticular outdoor systems.

LIST OF DESIGNATIONS 1 Electrode, electrical conductor 2 Holder,encapsulation 3 Insulator 6, 6′ Protective body 7 Gas area 8 Gap 10, 20Electrodes 12 Electrically conductive particle T Triple point U Voltagesource

What is claimed is:
 1. An electrical insulator for supporting anelectrode, which carries high voltage, on a holder having an insulatorbody and at least one protective body which at least partially forms thesurface of the insulator, wherein the protective body contains a porousstructure which has a mean dielectric constant which is less than thatof the material of the insulator body in order to avoid an excessivefield peak in the region of an electrically conductive particle locatedon the surface of the insulator, and the protective body has a thicknesscorresponding to the maximum length of the particles to be considered.2. The insulator as claimed in claim 1, wherein the material of theprotective body has a dielectric constant which is less than 2.5.
 3. Theinsulator as claimed in claim 1, wherein the protective body has athickness of at least 1 mm.
 4. The insulator as claimed in claim 1,wherein the protective body is flexible.
 5. The insulator as claimed inclaim 1, wherein the protective body contains a porous foam.
 6. Theinsulator as claimed in claim 5, wherein the foam is a syntactic foam.7. The insulator as claimed in claim 1, wherein the protective bodycontains an aerogel.
 8. The insulator as claimed in claim 1, wherein theprotective body has many thin walls, whose thickness is in each caseless than a typical length of the electrically conductive particles. 9.The insulator as claimed in claim 8, wherein the protective body is inthe form of a honeycomb.
 10. The insulator as claimed in claim 1,wherein the protective body is bonded to the insulator.
 11. Theinsulator as claimed in claim 1, wherein the protective body is at leastone of: arranged in a region in which the insulator is adjacent to theholder and arranged in a region in which the insulator is adjacent tothe electrode.
 12. The insulator as claimed in claim 1, wherein theprotective body is at least one of: at least partially arranged betweenthe insulator body and the holder, and at least partially arrangedbetween the insulator body and the electrode.
 13. The insulator asclaimed in claim 1, wherein the protective body extends along at leastone of: a part of the electrode and a part of the holder.
 14. Theinsulator as claimed in claim 1, wherein the holder is electricallyconductive.
 15. The insulator as claimed in claim 1, wherein thematerial of the protective body has a dielectric constant which is lessthan 1.5.
 16. The insulator as claimed in claim 1, wherein the wallshave a thickness of less than 1.5 mm.